Working principle of storage type digital oscilloscope and its limitations

This article illustrates the method of conversion of analogue signal into digital form and its storage in memory cell for its reconstruction at later stage by retrieving it from memory cell. The sampling technique in also illustrated with help of basic circuit diagram.

There are broadly two types of oscilloscope named as analogue oscilloscope and digital oscilloscope where the difference between both is well ill-starred by their name itself. Analogue oscilloscope involves only analogue signal during the processing of the signal in its various section like amplification, attenuation, filtration, deflection of any other operation some of its examples are sampling oscilloscope, conventional low frequency oscilloscope. But it is to mention here that it does not mean that it is not able to display digital signal as any wave of input signal can be studied by them irrespective of the types of oscilloscope used with some exceptions. On the other hand digital oscilloscope uses digital processing techniques in its various sections of processing like storage, transition between different sections. It is the availability of electronic circuits at relatively low cost that many digital features can be added to the analogue oscilloscopes like their storage. Some of the examples of such digital features involve a trigger after an elapse time or it may also be after a fixed number of pulses have passed. Digital display have some parameters like
• Integrated digital voltmeter
• Digital counter
• Remote control

In spite of the digital features embedded in it the basic system still remains an analogue oscilloscope on the basis of its working with some exceptions. Like an analogue oscilloscope CRT is used in it. The difference lies in the fact that it digitizes the analogue signal and as result all the substantial signals occur to be in digital form. As also has been told above that a conventional CRT is used here while storage of signal takes place in digital memory cells. The following figure well describes the working of basic oscilloscope with the help of a block diagram. This is basically a complete digital oscilloscope. The signal that is encountered by the system generally occurs in analogue form which is first digitized at input section and then it is stored in memory in the same form. Being in digital form various types of analysis can be done upon it and many other type of information can be built with this system and this all is possible because of the digital form of the given signal because in digital form an information signal can be easily fed into a computer and analysed by conventional methods.

As conventional CRTs work upon analogue signal only so in order to use CRT the signal must be converted back to its analogue form before feeding it to CRT. Therefore, the digital signal is first retrieved from memory cell then converted back to analogue form by using a conventional digital to analogue converter then is applied to CRT for its reproduction as a display.
Block diagram of digital storage type oscilloscope

Block diagram of digital storage type oscilloscope

The process of conversion from analogue to digital form is called Digitizing. Digitizing of the analogue signal takes place by use of sampling form. According to sampling theorem the samples of the input wave forms are taken out at fixed intervals of time which is further quantized to give a series of digital number equivalent to the magnitude of those samples. To make sure that no portion of the information signal is going to be lost the sampling rate should be greater than the frequency of the signal.

What is sampling frequency?

Sampling frequent is the rate at which the samples are taken out from input analogue signal in order to digitize the given information.
Sampled input signal and quantized levels

Sampled input signal and quantized levels

And according to Nyquist rate the sampling frequency should be equal to double of that of the input analogue signal. This rate ensures that no information is lost during digitizing process. This requirement for higher frequency rate implies the digitizer, which is an analogue to digital converter, must have a fast conversion rate. A diagram can illustrate the working of sampling technique in which samples of analogue wave form are being taken at fixed intervals of time denoted by points a,b,c etc. the higher sampling needs expensive flash analogue to digital converter and resolution of such converters falls with increase in sampling rate. This is the basic reason that band width and resolution of the system is kept limited to by the converter section.

There is absolution to overcome the need of high performance converter which is the use of an analogue store. The following figure shows one of such a kind. The sampling of input signal is performed and these samples are then stored in a shift register.
Block diagram of analogue storage type oscilloscope

Block diagram of analogue storage type oscilloscope

This shift register is of analogue nature. When it is required to display the data on CRT it can be directly fed to CRT without having need of conversion from analogue to digital form and this saves the time of conversion and this result in fast processing. The speed of this process is about 100 mega samples per second and this has an advantage that now a cheap analogue to digital converter can be utilized. Resolution of the converter does not fall with increase in sampling rate.

What are the Limitations of digital storage type oscilloscope?

As, all the techniques have its own advantages and disadvantage it also has some limitation that oscilloscope does not take the input signal when the digitizing process is going on. This results in a blind spot in this time period. When sweep speed is less then it is switched out analogue memory and deeds the analogue signal to convector the same time of operation. Many input channels can be used with digital storage oscilloscope. If all the channels are made to share the same store them memory allotted to each channel gets reduced. Sharing of many channel simultaneously is performed with the help of a multiplexer which a sequential circuit that makes it possible to uses same device by many user.
Input channel of such an oscilloscope can reach up to 40 channels per system which is very easily available in commercial market. This type of instrument is not used in small level laboratories but at commercial level laboratories and large research centres. The storage capacity of such a device ranges up tp 25000 dots. A floppy disc can be used in many such advanced oscilloscope as a storage device that leads to increment to the capability of the system as a whole. Floppy disc plays a role of external storage device and the memory stored upon it can be saved by the user.

Reconstruction of the wave form from memory cells

After, having the data stored in a memory section now it can be recalled to produce its wave form upon display screen. The reconstruction involves a large circuitry that can be studied in reconstruction process of analogue signal from digital signal.

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